The commercial applications of carbon nanotubes (CNTs) are numerous, but the creation of commercially viable products capable of sustaining industry is still a work in progress. In the pursuit of this goal, researchers are working to create CNTs with properties needed for individual CNT applications (e.g., transistors, heat sinks), and CNTs with properties suitable for assembly into macroscopic materials (e.g., yarns, sheets, and composites) that have high thermal conductivity, electrical conductivity, and mechanical strength. The consistent production and availability of the application-specific, ideal CNTs in laboratory and industrial scale quantities remains a challenge. Additionally, assembling CNTs in a fashion that yields macroscale properties even closely comparable to the properties of their constituents (i.e., the individual CNTs) also remains a challenge.
In the growth of carbon nanotubes using, for example, chemical vapor deposition (CVD), the length of the CNTs is relatively simple to control. However, carbon nanotubes, as grown, contain defects. These defects compromise the quality of the carbon nanotubes in regards to their thermal and electrical conductivity, as well as their mechanical integrity. Moreover, improved CNT alignment, packing density, and the presence of RBM bands in the Raman spectra are important to improve the thermal conductivity of CNT materials and to achieve a CNT superfiber. It is crucial to the future viability of the industry that these parameters are either corrected or controlled to deliver the most effective product. In addition, CNTs may be single-walled nanotubes (SWNTs) or multi-walled nanotubes (MWNTs). The carbon nanotubes are difficult to control with respect to the wall-count during the growth process. A routine process may be used to grow large quantities of relatively long multi-walled carbon nanotubes. However, carbon nanotubes synthesized with conventional MWCNT growth and heat treatment processes inhibit the liquid-state assembly that is likely needed to realize individual CNT properties in macroscopic materials (e.g., fibers and sheets) due to the properties offered (e.g., mechanical, electrical, thermal, and wall-count) and the limited attainable purity (e.g., residual catalyst, non-nanotube carbon, etc.).
Thermal treatment of CNTs aims to reduce the defects that limit the engineering properties of the tubes. One thermal treatment process includes heating CNTs to up to 2800° C., which is the phase transition temperature of graphite, within a high vacuum. During such a thermal treatment process, defects incurred during the growth process of the CNTs can be eliminated, along with graphitic impurities, as a function of the annealing temperature. Additionally, the tube begins to shed its walls over that temperature by the process of sublimation. Thus, CNT diameter and wall-count are a factor when considering the aggregate properties of CNT macrostructures. Such a process has value in that it can transform relatively low-value, high-defect, MWCNTs into low-diameter, high-quality CNTs with properties more amenable to high-performance applications regardless of the final form (e.g., array, fiber, sheet, composite, etc.).
One method of heating CNTs includes using a DC current. If a CNT is connected to two electrodes and subjected to a suitable DC current, the tube would increase in temperature as a result of Joule heating. The effect of this technique is mainly dominated by Joule heating and Fourier's law of conduction: Joule heating is the source and Fourier's law determines the temperature profile. If a CNT yarn or sheet is held stationary between the two electrodes and the temperature at a point rises above that of which the carbon atoms will remain in the CNT molecules, sublimation will begin.
However, DC annealing/sublimation is not a satisfactory solution for a variety of reasons. Considering the paradigms of the polymer production industry, which is likely the path that the CNT superfiber will follow, a liquid processing technique must be employed to fully realize the potential of these carbon polymers. In order to do this, the CNTs must be heat-treated prior to the liquid spinning process. The DC annealing techniques are inadequate due to the extremely high temperatures of the material under process (e.g., greater than 2800° C.) and the impracticality of uniform application of DC to an array of CNTs (i.e., an areal growth of what may seem to be uniform length but is, in fact, an agglomeration of billions of CNTs following a length distribution resembling a Gaussian distribution).
Thus, there is an increasing need to provide a solution capable of producing and processing improved carbon nanotubes that address one or more of the above drawbacks. For example, if a suitable CNT raw material may be produced, liquid-state spinning on CNTs with optimal morphology may, if properly performed, result in superior quality CNT superfibers.